Transfer cases are generally well known devices for providing a truck, automobile, or other vehicle with four-wheel drive capability. These four-wheel drive transfer cases include a housing which rotatably supports an input shaft for receiving torque from the primary transmission of the vehicle, as well as front and rear output shafts for transferring torque from the input shaft to the front and rear wheels of the vehicle, respectively. The transfer case may be provided with shifting means controllable by the driver for shifting the transfer case between the two-wheel and four-wheel drive modes as driving conditions warrant. More sophisticated transfer cases, such as "on-demand" four-wheel drive transfer cases, include automatic shifting means for automatically shifting the transfer case between two-wheel and four-wheel drive modes in response to variations in driving conditions. These transfer cases and shifting systems include complex electronic sensing systems for monitoring vehicle and road wheel speeds and other parameters such that the transfer case can be automatically shifted between two-wheel and four-wheel drive modes in response to the monitored vehicle conditions. Also, full-time four-wheel drive transfer cases may include an interaxle differential between the front wheels and the rear wheels of the motor vehicle.
Unlike part time four-wheel drive systems, a full-time four-wheel drive transfer case with an interaxle differential allows the front wheels and the rear wheels to rotate at different speeds with constant torque split, which occurs during normal operating conditions when, for example, turning the motor vehicle or in the event that the front wheels and the rear wheels have tires with different diameters. However, to prevent excessive slipping or differentiation between the front and the rear wheels, as might occur when one set of wheels encounters a low-traction condition such as ice, these transfer cases typically include a selectively engageable clutch which is operative to lock the interaxle differential upon sensing a predetermined amount of slippage. Locking of the interaxle differential prevents any relative slip or differentiation between the front output shaft and the rear output shaft of the transfer case. The "full-time" four wheel drive transfer cases may include systems for controlling the clutch assembly in a more sophisticated fashion to quickly "modulate" or regulate the speed of the vehicle wheels by varying the torque being transferred to the front and rear wheels of the vehicle to vary the handling characteristics of the vehicle during turns and other maneuvers. Known "full-time" four-wheel drive systems require electronic speed sensors positioned at the wheel ends or elsewhere to sense the angular velocity of each road wheel or output shaft such that the variation in angular velocity between the front and rear wheels may be monitored, with an excessive amount of variation in rotational speed being indicative of a low traction condition at the faster rotating wheel.
Generally, in a basic four-wheel drive transfer case, the input shaft receives torque directly from the primary transmission of the vehicle. The input shaft is connected to a rear output shaft of the transfer case such that the rear output shaft receives torque directly from the input shaft. Alternatively, the input shaft may be provided as one long shaft that travels through the transfer case, thus acting as the input and rear output shafts. The rear output shaft extends outward from the rear of the transfer case housing such that it may provide the input torque to the differential assembly of the rear wheels.
The front drive shaft of a four-wheel drive transfer case is offset from and parallel to the rear output shaft. The front drive shaft extends outward through the front of the transfer case housing and is connected to a differential that provides torque to the front road wheels of the vehicle. A drive sprocket is rotatably mounted about the rear output shaft of the transfer case and a clutch mechanism is provided for selectively coupling the drive sprocket to the rear drive shaft such that the sprocket rotates therewith. The front output shaft includes a sprocket that is connected to rotate therewith at all times, and a drive chain, belt, or the like is connected between these two sprockets such that torque is transferred to the front output shaft of the transfer case any time the four-wheel drive clutch assembly is actuated. Thus, two-wheel drive mode (torque being transferred to the rear wheels of the vehicle only) is provided when the clutch assembly is disengaged, and four-wheel drive mode (torque being transferred to all four vehicle wheels) is provided when the clutch is engaged. As an alternative to a drive chain or belt, one or more gears may be used to transfer torque to the front output shaft.
Recently, there has been an ever increasing demand for cars and trucks including four-wheel drive systems such as those described above. Furthermore, the purchasing public is demanding increasingly complex transfer cases that provide superior performance and durability without being unduly expensive. As described above, known transfer cases rely on speed sensors to provide data to the transfer case control system to control the split of torque between the front and rear output shafts. Such transfer cases do not include torque sensors which are connected to a torque monitoring and control system for monitoring the radial bearing load force on the bearings of the front and/or rear output shafts of the transfer case and converting the magnitude of the sensed bearing load into a related torque value that may be utilized by the control system to control the amount of torque transmitted to the shaft being monitored. Furthermore, the torque monitoring and control system may be utilized to limit the torque being transferred to the relevant shaft when the sensed bearing load approaches a predetermined threshold to thus prevent an overload of the shaft and its related components.